The present invention relates to the field of image quality correction circuits for correcting the tone of video images using a histogram of video signals.
Larger and wider screens are becoming increasingly common in color television sets, and much research attention is being focused on improvements in picture quality technology to obtain video images with higher picture quality.
A technology for applying a histogram of video signals is disclosed in the Japanese Laid-open Patent No. H5-66751. The disclosed technology emphasizes picture levels, i.e. levels of the luminance signal, occupying a large portion of the video signal. This technology improves the image quality and attempts to produce high-contrast video images.
An example of tone correction circuits using a histogram of the aforementioned patent is explained next with reference to figures.
FIG. 6 shows a block diagram of a tone correction circuit using the histogram of the aforementioned patent. In FIG. 6, a histogram generator 1 creates a histogram b which indicates the distribution of video signal levels of input video signals a. A video signal correction circuit 4 receives the histogram output b from the histogram generator 1, and creates and normalizes a cumulative histogram. The video signal correction circuit 4 further corrects the tone of the input video signals a using the normalized data, and outputs corrected video signals e.
The operation of the tone correction circuit using the histogram of the prior art as configured above is explained next.
In the circuit shown in FIG. 6, the histogram generator 1 creates, for example, a histogram with four intervals. In this case, if an image of a person standing against a dark background as shown in FIG. 8 is the input, then the histogram output from the histogram generator 1 represents frequencies H1, H2, H3, and H4 corresponding to intervals S1, S2, S3, and S4 as shown in FIG. 7. In FIG. 7, the input video signal level S is plotted along the abscissa and the frequency F is plotted along the ordinate.
More specifically, H1 is a frequency of the picture level falling in S1. Similarly, H2 is a frequency of the picture level falling in S2, H3 is a frequency of the picture level falling in S3, and H4 is a frequency of the picture level falling in S4.
The video signal correction circuit 4 cumulates and normalizes the histogram created as above to produce a cumulative and normalization function such as L1 in FIG. 7. The abscissa T shows the level of the cumulative and normalization function having a maximum value of 1. The video signal correction circuit 4 corrects and outputs the picture level of the input video signals using the cumulative and normalization function L1.
In other words L1 in FIG. 7 shows the characteristic of the input picture level versus the output picture level. Tone correction is achieved using a look-up table created from this characteristic. The xe2x80x98look-up tablexe2x80x99 hereafter refers to the input/output picture level characteristic represented, for example, by the cumulative and normalization function L1 in FIG. 7. If the slope of the cumulative and normalization function L1 is steep, the ratio of the output picture level to the input picture level increases, which means that the tone is intensified.
For example, consider the case where video signals are concentrated in picture level S1, and therefore the frequency H1 becomes large. If after cumulation and normalization a look-up table is produced for correction, then the slope of cumulative and normalization function L1 corresponding to the interval S1 becomes steep. Thus, the contrast of the picture level in the interval S1, where the video signals are concentrated is increased. Accordingly, the tone correction circuit using a histogram of the aforementioned patent executes tone correction without modifying the original histogram.
With the above circuit configuration, however, parts of the image important to a user may be reproduced at a lower than optimal tone. This problem can arise in parts of the image a user wishes to pay particular attention to, such as human faces, because this part of the image only occupies a small area which is attenuated by histogram analysis.
A tone correction circuit of the present invention corrects the tone of video signals by using a histogram. The histogram is in turn corrected again by a controller which is adjustable. The controller is adjusted as required for intensifying the input/output video signal characteristic of the picture level in certain areas of importance to the user. Thus, the tone of video signals is corrected using corrected histograms.
Accordingly, the present invention offers a tone correction circuit for video signals that corrects the tone by intensifying the contrast of an area which the user may want to pay particular attention to such as human faces.